Solid electrolyte

ABSTRACT

The solid electrolyte of the present invention is composed of an organic/inorganic composite material having pores with a mean pore diameter of 1 to 30 nm and having a skeleton comprising a metal atom, an oxygen atom bonded to the metal atom, and an organic group having at least one carbon atom bonded to the metal atom or the oxygen atom, and a functional group having an ion exchange function and bonded to the organic group inside the pores. As a result, even if the relative pressure of the water vapor in the atmosphere is less than 1.0, it is still possible to achieve a solid electrolyte with a sufficiently high ion conductivity at a lower temperature than with a conventional solid electrolyte such as stabilized zirconia.

DESCRIPTION

[0001] 1. Technical Field

[0002] This invention relates to a solid electrolyte, and moreparticularly relates to a solid electrolyte with a porous structure.

[0003] 2. Background Art

[0004] Studies have been conducted in the past into the application of asolid electrolyte having ion conductivity, such as stabilized zirconia,lithium nitride, or β-alumina, to the electrolytic membranes of fuelcells, the electrolyte used in completely solid cells, sensors, and soforth. The ion conductivity of these solid electrolytes is believed tobe the result of the movement of ions through the lattice or defects ofa solid.

[0005] Also, there are polymer electrolytes such as perfluorosulfonicacid or hydrocarbon-based polymers, which have been studied aselectrolytic membranes for solid polymer type fuel cells, and it isknown that when these are wetted with an electrolytic solution such aswater, ions move through the electrolytic solution present in the voidsof the polymer chain, resulting in ion conductivity. These are calledquasi-solid electrolytes. Among the advantages to these polymerelectrolytes are that they exhibit ion conductivity at a relatively lowtemperature, they are easy to mold into thin films and so forth, andthey provide good contact with electrodes, and for these reasons theyare very promising as electrolytic membranes for fuel cells.

[0006] 3. Disclosure of the Invention

[0007] However, with conventional solid electrolytes, a large amount ofactivation energy is usually needed for moving the ions through thelattice or defects, so these solid electrolytes do not necessarilyexhibit sufficient ion conductivity under low-temperature conditions.Consequently, a problem is that they have to be kept at a hightemperature (at least 700° C. in the case of stabilized zirconia, forexample) with a heating apparatus or the like in order for sufficientlyhigh ion conductivity to be obtained.

[0008] Meanwhile, conventional polymer electrolytes usually exhibit ionconductivity at a lower temperature than the other conventional solidelectrolytes discussed above, but adequate ion conductivity is notalways exhibited in a state in which the electrolytic solution does notfully fill the voids in the polymer chain. Accordingly, when a polymerelectrolyte is used, the vapor pressure of the electrolytic solution hasto be kept at the saturated vapor pressure. For example, when water isused for the electrolytic solution, a humidifier or the like has to beused to keep the relative pressure of the water vapor in the atmosphereat 1.0 (that is, a relative humidity of 100%).

[0009] The present invention was conceived in light of the aboveproblems encountered with related art, and it is an object thereof toprovide a solid electrolyte that exhibits sufficiently high ionconductivity at a lower temperature than with a conventional solidelectrolyte such as stabilized zirconia, even if the relative pressureof the water vapor in the atmosphere is less than 1.0.

[0010] As a result of diligent research aimed at achieving the statedobject, the inventors perfected the present invention upon discoveringthat the above problems can be solved if an organic/inorganic compositematerial having pores whose mean pore diameter is within a specificrange is used for the material of the electrolytic solution, andfunctional groups having an ion exchange function are bonded to theorganic groups that make up the skeleton of this organic/inorganiccomposite material.

[0011] Specifically, the solid electrolyte of the present invention haspores with a mean pore diameter of 1 to 30 nm, comprising:

[0012] an organic/inorganic composite material having:

[0013] a skeleton comprising a metal atom, an oxygen atom bonded to themetal atom, and an organic group having at least one carbon atom bondedto the metal atom or the oxygen atom; and

[0014] a functional group having an ion exchange function and bonded tothe organic group inside the pores.

[0015] With the present invention, an organic/inorganic compositematerial having a skeleton with the above-mentioned characteristics andhaving pores whose mean pore diameter is within the above-mentionedspecified range is used as the material for the solid electrolyte, andfunctional groups having an ion exchange function are bonded to organicgroups inside the pores of this organic/inorganic composite material,the result being that even if the relative pressure of the water vaporin the atmosphere is less than 1.0, the pores will still be fully filledwith water through capillary action. Within the pores thus fully filledwith water, the functional groups having an ion exchange function makeit possible for the ions in the water to be sufficiently conductedthrough the same ion conduction mechanism as with a polymer electrolyte.Therefore, even if the relative pressure of the water vapor in theatmosphere is less than 1.0, it is still possible to obtain sufficientlyhigh ion conductivity at a lower temperature than with a conventionalsolid electrolyte such as stabilized zirconia.

[0016] The term “relative pressure” as used in the present inventionrefers to the ratio p/p₀between the vapor pressure p of the solvent at aspecific temperature and the saturated vapor pressure p₀. For instance,in the case of water vapor, a relative pressure of 1.0 is defined to bethe same as a relative humidity of 100%.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a graph of the X-ray diffraction patterns obtained forthe solid electrolyte and porous particles in Example 1;

[0018]FIG. 2 is a graph of the X-ray diffraction patterns obtained forthe solid electrolyte and porous particles in Example 1;

[0019]FIG. 3 is a graph of the nitrogen adsorption isotherms obtainedfor the solid electrolyte and porous particles in Example 1;

[0020]FIG. 4 is a graph of the pore diameter distribution curve obtainedfor the solid electrolyte and porous particles in Example 1;

[0021]FIG. 5 is a sodium hydroxide titration graph obtained for thesolid electrolyte in Example 1, in which a represents the correlationbetween the titer of a 0.05N sodium hydroxide aqueous solution and thepH, and b represents the correlation between the titer of a 0.05N sodiumhydroxide aqueous solution and d (pH)/dV (pH) (the value obtained bydifferentiating the pH with the titer V); and

[0022]FIG. 6 is a graph of the water vapor adsorption isotherms at 25°C. obtained for the solid electrolytes in Example 1 and ComparativeExample 2 and for the porous particles in Comparative Example 2.

BEST MODES FOR CARRYING OUT THE INVENTION

[0023] Preferred embodiments of the present invention will now bedescribed in detail.

[0024] The organic/inorganic composite material used in the presentinvention has pores with a mean pore diameter of 1 to 30 nm, and has askeleton comprising a metal atom, an oxygen atom bonded to the metalatom, and an organic group having at least one carbon atom bonded to themetal atom or the oxygen atom, and a functional group having an ionexchange function and bonded to the organic group inside the pores.

[0025] In the organic/inorganic composite material pertaining to thepresent invention, the mean pore diameter is 1 to 30 nm, as mentionedabove, and is preferably 1 to 10 nm. A mean pore diameter of over 30 nmwill hinder capillary action, and the pores cannot be fully filled withelectrolytic solution even if the relative pressure of the electrolyticsolution in the atmosphere is less than 1.0. Also, capillary actiongenerally occurs more readily as the median diameter of the pores isreduced, but if the mean pore diameter is less than 1 nm, theelectrolytic solution will be in a form closer to a solid than a liquid,and the ion conductivity will tend to be inadequate. The relationbetween the relative pressure (p/p₀) and the pore diameter (D) at whichcapillary action occurs is expressed by the following equation (Kelvin'sequation).

1n(p/p ₀)=−(2γV _(L) cos θ)/{(D/2)RT}

[0026] where γ is the surface tension of the condensed liquid, V_(L) isthe molar molecular volume of the condensed liquid, θ is the contactangle between the pore walls and the condensed liquid, R is a gasconstant, and T is the absolute temperature.

[0027] It can be seen from the above equation that the smaller is thepore diameter, the lower is the relative pressure at which capillaryaction will occur.

[0028] The “mean pore diameter” referred to here is the pore diameter atthe maximum peak of the curve (pore diameter distribution curve)obtained by plotting the value (dV/dD) obtained by differentiating thepore volume (V) with the pore diameter (D) against the pore diameter(D). A pore diameter distribution curve can be produced by the followingmethod. The organic/inorganic composite material is cooled to thetemperature of liquid nitrogen (−196° C.), nitrogen gas is introduced,the amount of adsorption thereof is found by a constant volume or weightmethod, then the pressure of the introduced nitrogen gas is slowlyincreased, the amount of nitrogen gas adsorption is plotted against thevarious equilibrium pressures, and an adsorption isotherm is obtained.This adsorption isotherm can be used to find the pore diameterdistribution curve by the Cranston-Inklay method, the Pollimore-Healmethod, the BJH method, or another such calculation method.

[0029] The organic/inorganic composite material pertaining to thepresent invention is preferably such that at least 60% of the total porevolume is included in the range of ±40% of the mean pore diameter on thepore diameter distribution curve. The phrase “at least 60% of the totalpore volume is included in the range of ±40% of the mean pore diameteron the pore diameter distribution curve” as used here means that if themean pore diameter is 3.00 nm, for example, the combined volume of poreswithin ±40% of this 3.00 nm, that is, within a range of 1.80 to 4.20 nm,accounts for at least 60% of the total pore volume. What this means isthat an organic/inorganic composite material that satisfies thisrequirement has an extremely uniform pore diameter.

[0030] There are no particular restrictions on the specific surface areaof the organic/inorganic composite material pertaining to the presentinvention, but at least 700 m²/g is preferable. The specific surfacearea can be calculated as the BET specific surface area from theadsorption isotherm using a BET isotherm adsorption formula.

[0031] It is also preferable for the organic/inorganic compositematerial pertaining to the present invention to have at least one peakfor diffraction angle corresponding to a d value of at least 1 nm in theX-ray diffraction of this material. An X-ray diffraction peak means thatthere is a periodic structure of the d value corresponding to the peakangle thereof in the sample. Therefore, one or more peaks in thediffraction angle corresponding to a d value of 1 nm or more means thatthe pores are regularly arranged at a spacing of at least 1 nm.

[0032] The pores of the organic/inorganic composite material pertainingto the present invention are formed not only on the surface of theparticles, but also in the interior. There are no particularrestrictions on the shape of these pores, but they may, for example, gothrough the material in the form of tunnels, or they may consist ofspherical or polyhedral cavities linked together.

[0033] As discussed above, the organic/inorganic composite materialpertaining to the present invention has a skeleton comprising a metalatom, an oxygen atom bonded to the metal atom, and an organic grouphaving at least one carbon atom bonded to the metal atom or the oxygenatom. Examples of this skeleton include the following (a) and (b).

[0034] (a) A skeleton composed of an organic group having one or morecarbon atoms, two or more metal atoms bonded to the same or differentcarbon atoms in the organic group, and one or more oxygen atoms bondedto the metal atoms (hereinafter referred to as “organic/inorganic hybridskeleton”).

[0035] (b) An inorganic skeleton composed of a metal atom and an oxygenatom bonded to this metal atom, in which an organic group having one ormore carbon atoms is bonded to the metal atom or the oxygen atom(hereinafter referred to as “surface modified organic/inorganiccomposite skeleton”).

[0036] The organic group in the organic/inorganic hybrid skeleton musthave a valence of at least 2 in order to bond with the two or more metalatoms. Examples of such organic groups include divalent or higherorganic groups produced when two or more hydrogen atoms are removed froman alkane, alkene, alkyne, benzene, cycloalkane, or other suchhydrocarbon. The organic/inorganic hybrid skeleton pertaining to thepresent invention may include just one type of the above-mentionedorganic group, or may contain two or more types.

[0037] In the present invention, it is preferable for the valence of theorganic group to be 2 because this will give an organic/inorganiccomposite material with the proper degree of crosslinking. Examples ofdivalent organic groups include a methylene group (—CH₂—), ethylenegroup (—CH₂CH₂—), trimethylene group (—CH₂CH₂CH₂—), tetramethylene group(—CH₂CH₂CH₂CH₂—), 1,2-butylene group (—CH (C₂H₅) CH—), 1,3-butylenegroup (—CH (CH₃) CH₂CH₂—), phenylene group (—C₆H₄—), diethylphenylenegroup (—C₂H₄—C₆H₄—C₂H₄—), vinylene group (—CH═CH—), propenylene group(—CH₂—CH═CH₂—), butenylene group (—CH₂—CH═CH—CH₂—), amide group(—CO—NH—), dimethylamino group (—CH₂—NH—CH₂—), and trimethylamine group(—CH₂—N(CH₃)—CH₂—). Of these, a methylene group, ethylene group, orphenylene group is preferred because they allow porous particles with ahigh crystallinity to be obtained.

[0038] Two more metal atoms are bonded to the same or different carbonatoms in the above-mentioned organic group, and while there are noparticular restrictions on the type of these metal atoms, examplesinclude silicon, aluminum, titanium, magnesium, zirconium, tantalum,niobium, molybdenum, cobalt, nickel, gallium, beryllium, yttrium,lanthanum, hafnium, tin, lead, vanadium, and boron. Of these, silicon,aluminum, and titanium are preferred because bonding will be better withthe organic group and the oxygen. The above-mentioned metal atoms forman oxide by bonding with the organic group and bonding with an oxygenatom, and this oxide maybe a compound oxide consisting of two or moretypes of metal atom.

[0039] The organic/inorganic hybrid skeleton is formed by the bonding ofthe above-mentioned organic group, metal atoms, and oxygen atom, but thetype of these bonds is not limited, and examples include covalent bondsand ion bonds. Organic/inorganic composite materials with differentskeletons (linear, ladder-shaped, reticulated, branched, etc.) will beproduced depending on the number of metal atoms bonded to the organicgroup, and the oxygen atoms bonded to the metal atoms.

[0040] In the organic/inorganic hybrid skeleton, since the organic groupis bonded to two or more metal atoms, and these metal atoms are bondedto one or more oxygen atoms, the organic group is incorporated into theskeleton of a metal oxide. As a result, the organic/inorganic compositematerial pertaining to the present invention exhibits the surfacecharacteristics of both an organic and an inorganic material.

[0041] Of these organic/inorganic hybrid skeletons, one composed of atleast one type of structural unit expressed by the following GeneralFormula (1) is preferable.

[0042] In Formula (1) above, R¹ is an oxygen atom with at least onecarbon atom, and M is a metal atom. Specific examples of R¹ and M arethe groups or atoms listed above in the description of the oxygen atomand metal atom.

[0043] In Formula (1) above, R² is a hydrogen atom, a hydroxyl group, ora hydrocarbon group. If R² is a hydrocarbon group, there are norestrictions on the type thereof, but examples of R²include C₁ to C₁₀alkyl groups, C₁ to C₁₀ alkenyl groups, a phenyl group, and substitutedphenyl groups.

[0044] x in the above General Formula (1) is an integer obtained bysubtracting 1 from the valence of the metal M, n is an integer greaterthan or equal to 1 and less than or equal to x, and m is an integergreater than or equal to 2. The carbons of R¹ to which M is bonded maybe the same or different. —O_(1/2)— here indicates a group that becomes—O— when two of these groups are bonded.

[0045] When R¹, M, R², n, and m in General Formula (1) above are anethylene group, silicon atom, methyl group, 1, and 2, respectively,General Formula (1) is expressed by the following Chemical Formula (2).

[0046] A skeleton in which two of the structural units of the aboveChemical Formula (2) are linked is expressed by the following ChemicalFormula (3).

[0047] When R¹, M, n, and m in General Formula (1) above are an ethylenegroup, silicon atom, 3, and 2, respectively, General Formula (1) isexpressed by the following Chemical Formula (4).

[0048] When a plurality of the structural units of the above ChemicalFormula (4) are linked, a reticulated structure is formed. The followingChemical Formula (5) expresses a case in which four of the structuralunits of the above Chemical Formula (4) are linked, as an example ofthis reticulated structure.

[0049] The organic/inorganic hybrid skeleton pertaining to the presentinvention may be composed of a plurality of structural units in whichR¹, M, R², n, and m in General Formula (1) above are different. Forexample, this skeleton may be composed of structural units expressed bythe above Chemical Formula (2) and structural units expressed by theabove Chemical Formula (4). When the organic/inorganic compositematerial pertaining to the present invention has structural unitsexpressed by the above General Formula (1) as an organic/inorganichybrid skeleton, in addition to these structural units, the material mayalso have structural units such as Si—(O_(1/2))₄— or Ti—(O_(1/2))₄—.

[0050] An organic/inorganic composite material having anorganic/inorganic hybrid skeleton can be obtained, for example, by thepolycondensation of at least one type of compound expressed by thefollowing general formula.

[0051] R¹, M, and R² here are the same as R¹, M, and R² in the aboveGeneral Formula (1), respectively. A is an alkoxy group or a halogenatom, x is an integer obtained by subtracting 1 from the valence of themetal M, n is an integer greater than or equal to 1 and less than orequal to x, and m is an integer greater than or equal to 1. The carbonsof R¹ to which M is bonded may be the same or different.

[0052] When A in the above General Formula (6) is an alkoxy group, thereare no restrictions on the type of hydrocarbon group bonded to theoxygen in that alkoxy group, but examples include linear, cyclic, andalicyclic hydrocarbons. This hydrocarbon group is preferably a C₁ to C₅linear alkyl group, with a methyl group or ethyl group beingparticularly favorable.

[0053] When A in the above General Formula (6) is a halogen atom, thereare no restrictions on the type thereof, but examples include a chlorineatom, bromine atom, fluorine atom, and iodine atom. Of these, chlorineand bromine are preferred.

[0054] When R¹, M, A, n, and m in General Formula (6) above are anethylene group, silicon atom, methoxy group, 3, and 2, respectively, thecompound expressed by General Formula (6) is1,2-bis(trimethoxysilyl)ethane, which is expressed by(CH₃O)₃Si—CH₂—CH₂—Si(OCH₃)₃.

[0055] When R¹, M, A, n, and m in General Formula (6) above are anethylene group, silicon atom, chlorine, 3, and 2, respectively, thecompound expressed by General Formula (6) is1,2-bis(trichlorosilyl)ethane, which is expressed byCl₃Si—CH₂—CH₂—SiCl₃.

[0056] In the present invention, an alkoxysilane, titanium alkoxide,aluminum alkoxide, or the like may be added by polycondensation to thecompound expressed by General Formula (6).

[0057] This alkoxysilane can be tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, or the like. An alkoxysilane having a functionalgroup such as an amino group, carboxyl group, mercapto group, or epoxygroup can also be used.

[0058] The titanium alkoxide can be titanium butoxide, titaniumisopropoxide, or titanium ethoxide, for example, and the aluminumalkoxide can be aluminum isopropoxide, for example. Various kinds ofmetal halide such as silicon chloride (SiCl₄) can also be used.

[0059] An SiO₂—Al₂—Al₂O₃ skeleton can be introduced into thealkoxysilane or compound expressed by the above General Formula (6) byadding and reacting pseudo-boehmite, sodium alumate, aluminumsulfate,dialkoxyaluminotrialkoxysilane, or the like. A skeleton of vanadium,boron, or manganese can be introduced by adding and reacting vanadylsulfate (VOSO₄), boric acid (H₃BO₃), manganese chloride (MnCl₂), or thelike.

[0060] In the manufacture of an organic/inorganic composite materialhaving an organic/inorganic hybrid skeleton, it is preferable to add acompound expressed by the above General Formula (6) to an aqueoussolution containing a surfactant, and perform polycondensation underacidic or alkaline conditions.

[0061] This surfactant can be an anionic, cationic, or nonionicsurfactant. Examples of this surfactant include a chloride, bromide,iodide, hydroxide, or the like of an alkyltrimethylammonium[C_(n)H_(2n+1)N (CH₃)₃], alkylammonium, dialkyldimethylammonium, orbenzylammonium, as well as fatty acid salts, alkylsulfonates,alkylphosphates, polyethylene oxide-based nonionic surfactants, andprimary alkylamines.

[0062] It is preferable for the alkyltrimethylammonium[C_(n)H_(2n+1)N(CH₃)₃] to be one in which the carbon number of the alkylgroup is from 8 to 18.

[0063] Examples of nonionic surfactants include polyoxyethyleneoxide-based nonionic surfactants having a hydrocarbon group as itshydrophobic component and having polyethylene oxide chain as itshydrophilic component. Examples of such surfactants includeC₁₆H₃₃(OCH₂CH₂)₂OH, C₁₂H₂₅(OCH₂CH₂)₄OH, C₁₆H₃₃(OCH₂CH₂)₁₀OH,C₁₆H₃₃(OCH₂CH₂)₂₀OH, C₁₈H₃₇(OCH₂CH)₁₀OH, C₁₈H₃₅(OCH₂CH₂)₁₀OH, andC₁₂H₂₅(OCH₂CH₂)₂₃OH.

[0064] A surfactant having a sorbitan fatty acid ester component and apolyethylene oxide component can also be used. Examples of suchsurfactants include Triton X-100 (Aldrich), polyethylene oxide (20)sorbitan monolaurate (Tween 20, Aldrich), polyethylene oxide (20)sorbitan monopalmitate (Tween 40), polyethylene oxide (20) sorbitanmonooleate (Tween60), and sorbitan monopalmitate (Span 40).

[0065] A tri-block copolymer composed of three polyalkylene oxide chainscan also be used as a surfactant. Of these, a tri-block copolymercomposed of a polyethylene oxide (EO) chain, a polypropylene oxide (PO)chain, and a polyethylene oxide (EO) chain is preferred. If we let x bethe number of EO chain repeating units and y be the number of PO chainrepeating units, this tri-block copolymer can be expressed by(EO)_(x)(PO)_(y)(EO)_(x). There are no particular restrictions on x andy in the tri-block copolymer used in the present invention, but it ispreferable for x to be 5 to 110 and y to be 15 to 70. It is particularlypreferable for x to be 15 to 20 and y to be 50 to 60.

[0066] A tri-block copolymer composed of a polypropylene oxide (PO)chain, a polyethylene oxide (EO) chain, and a polypropylene oxide (PO)chain ((PO)_(x)(EO)_(y)(PO)_(x)) can also be used favorably. There areno particular restrictions on x and y here, but it is preferable for xto be 5 to 110 and y to be 15 to 70, and even more preferable for x tobe 15 to 20 and y to be 50 to 60.

[0067] Examples of the above-mentioned tri-block copolymer include(EO)₅(PO)₇₀(EO)₅, (EO)₁₃(PO)₃₀(EO)₁₃, (EO)₂₀(PO)₃₀(EO)₂₀,(EO)₂₆(PO)₃₉(EO)₂₆, (EO)₁₇(PO)₅₆(EO)₁₇, (EO)₁₇(PO)₅₈(EO)₁₇,(EO)₂₀(PO)₇₀(EO)₂₀, (EO)₈₀(PO)₃₀(EO)₈₀, (EO)₁₀₆(PO)₇₀(EO)₁₀₆,(EO)₁₀₀(PO)₃₉(EO)₁₀₀, (EO)₁₉(PO)₃₃(EO)₁₉, and (EO)₂₆(PO)₃₆(EO)₂₆. Ofthese, it is preferable to use (EO)₁₇(PO)₅₆(EO)₁₇ and(EO)₁₇(PO)₅₈(EO)₁₇. These tri-block copolymers are available from BASFand elsewhere, and a tri-block copolymer having the desired x and yvalues can be obtained on a small-scale manufacturing level. Theabove-mentioned tri-block copolymers can be used singly or incombinations or two or more types.

[0068] A star-type block copolymer in which two polyethylene oxide (EO)chain/polypropylene oxide (PO) chain units are bonded to two nitrogenatoms in ethylenediamine, respectively, can also be used. Examples ofthis star-type block copolymer include ((EO)₁₁₃(PO)₂₂)₂NCH₂CH₂N((PO)₂₂(EO)₁₁₃)₂, ((EO)₃(PO)₁₈)₂NCH₂CH₂N ((PO)₁₈(EO)₃)₂, and((PO)₁₉(EO)₁₆)₂NCH₂CH₂N ((EO)₁₆(PO)₁₉)₂. The above-mentioned star-typeblock copolymers can be used singly or in combinations or two or moretypes.

[0069] An organic/inorganic composite material having anorganic/inorganic hybrid skeleton can be obtained by adding a compoundexpressed by the above General Formula (6) (and, if necessary, aninorganic compound such as an alkoxysilane) to an aqueous solutioncontaining a surfactant, and performing polycondensation under acidic oralkaline conditions, but it is preferable for the pH of the aqueoussolution to be 7 or higher.

[0070] Also, an organometal compound (and an inorganic compound ifneeded) can be subjected to polycondensation in the absence of asurfactant and under acidic or alkaline conditions to form an oligomer,and a surfactant can be added to an aqueous solution containing thisoligomer, and polycondensation again performed under acidic or alkalineconditions.

[0071] In the polycondensation performed in the presence of asurfactant, it is also possible for polycondensation under alkalineconditions and polycondensation under acidic conditions to be performedalternately. There are no particular restrictions on the order of thealkaline and acidic conditions here, but the degree of polymerizationwill tend to be higher if polycondensation is first performed underacidic conditions and then under alkaline conditions. Also, it ispreferable for the system to be alternately agitated and allowed tostand during the polycondensation reaction.

[0072] The polycondensation reaction temperature is preferably between 0and 100° C., but the lower the temperature, the more regular thestructure of the product will tend to be. The best reaction temperaturefor increasing the regularity of the structure is 20 to 40° C. On theother hand, the higher the reaction temperature, the more stable thestructure will tend to be. The best reaction temperature for raising thedegree of polymerization is 60 to 80° C.

[0073] After the polycondensation reaction, aging is performed, and theprecipitate or gel thus produced is filtered and, if needed, washed, andthen dried, which gives a porous material precursor in which the poresare still filled with surfactant.

[0074] This porous particle precursor can be dispersed in an aqueoussolution containing the same surfactant as that used in thepolycondensation reaction (typically the surfactant concentration is thesame as or lower than that during the polycondensation reaction) or inan electrolytic solution such as water, and the precursor subjected to awet heat treatment at 50 to 200° C. In this case, the system can beheated with or without the solution used in the polycondensationreaction being diluted. The heating temperature is preferably from 60 to100° C., with 70 to 80° C. being particularly favorable. The pH hereshould be weakly alkaline, and is preferably 8 to 8.5, for example.There are no particular restrictions on the duration of the wet heattreatment here, but an hour or longer is preferable, with 3 to 8 hoursbeing particularly good.

[0075] After this wet heat treatment, the porous material precursor isfiltered and dried to remove any excess treatment liquid. The porousmaterial precursor may be agitated at room temperature for anywhere froma few hours to a few dozen hours prior to being dispersed in theabove-mentioned aqueous solution or solvent, having its pH adjusted, andundergoing the wet heat treatment.

[0076] The surfactant is then removed from the porous materialprecursor, and examples of how this can be accomplished include baking,and treating with a solvent such as water or an alcohol.

[0077] When baking is employed, the porous particle precursor is heatedat 300 to 1000° C., and preferably 400 to 700° C. About 30 minutes islong enough for the heating, but heating for at least an hour ispreferable in order to completely remove the surfactant component. Thisbaking can be carried out in the air, but since a large amount ofcombustion gas is produced, it may also be performed while an inert gassuch as nitrogen is introduced.

[0078] When a solvent is used to remove the surfactant from the porousparticle precursor, for example, the porous material precursor isdispersed in a solvent with good surfactant solubility, and the systemis agitated to recover the solids. Water, ethanol, methanol, acetone, orthe like can be used as the solvent.

[0079] When a cationic surfactant is used, the porous material precursoris dispersed in water or ethanol to which a small amount of hydrochloricacid has been added, and the system is agitated under heating at 50 to70° C. This results in the cationic surfactant being ion-exchanged andextracted by protons. When an anionic surfactant is used, it can beextracted in a solvent to which an ion shave been added. When a nonionicsurfactant is used, it can be extracted with just a solvent. The systemis preferably ultrasonically bombarded during this extraction. Also,combining or repeating agitation and standing is preferable.

[0080] The shape of the organic/inorganic composite material pertainingto the present invention can be controlled by varying the synthesisconditions. The shape of the organic/inorganic composite materialreflects the structure in which the pores of the particles are arranged,and the shape is also determined by the crystal structure. For instance,the crystal structure of spherical particles is three-dimensionallyhexagonal, while the crystal structure of particles in the form ofhexagonal prisms is two-dimensionally hexagonal. The crystal structureof octadecahedral particles is cubic.

[0081] Examples of synthesis conditions that affect the shape (crystalstructure) of the organic/inorganic composite material include thereaction temperature and the length (carbon number) of the surfactant.For example, when an alkyltrimethylammonium is used as the surfactant,the carbon number of the alkyl groups thereof and the reactiontemperature affect the shape of the organic/inorganic compositematerial. For instance, when the reaction temperature is 95° C. and thecarbon number of the alkyl groups is 18, particles tend to be producedin the form of hexagonal prisms, but when the reaction temperature is95° C. and the carbon number of the alkyl groups is 16, octadecahedralparticles tend to be produced. When the reaction temperature is 25° C.,spherical particles tend to be produced whether the carbon number of thealkyl groups is 16 or 18. On the other hand, a laminar structure resultswhen the reaction temperature is 2° C. and the carbon number of thealkyl groups is 18, while spherical particles tend to be produced whenthe reaction temperature is 2° C. and the carbon number of the alkylgroups is 16.

[0082] Meanwhile, the surface modified organic/inorganic compositeskeleton (b) has a polymer main chain of an inorganic oxide made up ofmetal atoms and oxygen atoms. Examples of the metal atoms that make uppart of the main chain are the same as the metal atoms listed in theabove description of the organic/inorganic hybrid skeleton, and ofthese, silicon, aluminum, and titanium are preferred because they bondbetter with organic groups and oxygen. Furthermore, the metal atoms bondwith oxygen atoms to form an oxide in an surface modifiedorganic/inorganic composite skeleton, and this oxide may be a compoundoxide containing two or more types of metal atom. The main chain of aninorganic skeleton may be linear, branched, ladder-shaped, orreticulated.

[0083] Specific examples of the organic groups in the surface modifiedorganic/inorganic composite skeleton include a methyl group, ethylgroup, or other such C₁ to C₆ alkyl group; and a phenyl group or othersuch C₆ to C₁₂ aryl group. The bonding position of these organic groupsmay be either the oxygen atoms or the metal atoms that make up theinorganic skeleton.

[0084] There are no particular restrictions on the method formanufacturing an organic/inorganic composite material having a surfacemodified organic/inorganic composite skeleton, but when a silicateskeleton (—Si—O—) is formed, for example, [this material] can beobtained by subjecting an organosilane expressed by the followingFormula (7):

R—Si(OR′)₃  (7)

[0085] where R is a C₁ to C₆ alkyl group or a C₆ to C₁₂ aryl group, andR′ is a methyl group or ethyl group, and, if needed, an alkoxysilanesuch as tetramethoxysilane, tetraethoxysilane, or tetrapropoxysilane topolycondensation using a template surfactant, and then removing thesurfactant. Organic groups can also be introduced at the surface of theinorganic skeleton by subjecting the above-mentioned alkoxysilane or aninorganic skeleton component such as sodium silicate or kanemite(NaHSi₂O₅·3H₂O) to polycondensation using a surfactant, removing thesurfactant to obtain an inorganic porous material, and then reacting thesilanol groups (Si-OH) present on the surface of the inorganic skeletonwith the above-mentioned organosilane or a halogenated organosilane suchas trimethoxychlorosilane [Cl—Si—(OCH₃)₃].

[0086] It is also possible to form an inorganic skeleton containingaluminum by using pseudo-boehmite, sodium alumate, aluminumsulfate,dialkoxyaluminotrialkoxysilane, or the like. A metallosilicate-basedskeleton (SiO₂—MO_(n/2)) in which any of various metals (M^(n+); where Mis a metal such as titanium, zirconium, tantalum, niobium, tin, orhafnium) is included in a silicate skeleton can be obtained by using anoxide in which the silicon of the inorganic skeleton component given asan example in the formation of the above-mentioned silicate skeleton issubstituted with titanium, zirconium, tantalum, niobium, tin, orhafnium. More specifically, a metallosilicate porous material in whichtitanium, vanadium, boron, or manganese has been introduced can beobtained by adding Ti (OC₂H₅)₄ or another such titanate compound,vanadyl sulfate (VOSO₄), boric acid (H₃BO₃), or manganese chloride(MnCl₂), respectively, to an alkoxysilane and performing acopolymerization reaction.

[0087] The same surfactants as those given in the description of theorganic/inorganic hybrid skeleton can be used as a template in theformation of the surface modified inorganic/inorganic compositeskeleton, and the polycondensation and removal of the surfactant can beaccomplished in the same way as in the formation of theorganic/inorganic hybrid skeleton.

[0088] The solid electrolyte of the present invention has a structure inwhich functional groups having an ion exchange function are bonded toorganic groups inside the pores of the organic/inorganic compositematerial having the above structure, and exhibits sufficiently high ionconductivity at a lower temperature than with a conventional solidelectrolyte such as stabilized zirconia, even if the relative pressureof the water vapor in the atmosphere is less than 1.0.

[0089] The functional groups having an ion exchange function here have,in addition to a function of imparting ion conductivity to the solidelectrolyte of the present invention, a function of making it easier forthe pores to be filled with water or another such electrolytic solution.Specifically, capillary action will occur even if the functional groupshaving an ion exchange function are not disposed in the pores of theorganic/inorganic composite material pertaining to the presentinvention, but disposing the functional groups having an ion exchangefunction inside these pores in the solid electrolyte of the presentinvention allows the pores to be fully filled with electrolytic solutionunder conditions of lower relative pressure of the electrolyticsolution.

[0090] Specific examples of functional groups having an ion exchangefunction include sulfonic acid groups, phosphoric acid groups,carboxylic acid groups, and sulfonimide groups, but it is preferable forthis functional group to be at least one selected from the groupconsisting of sulfonic acid groups (—SO₃H), phosphoric acid groups(—PO₄H₂ or >PO₄H), and carboxylic acid groups (—COOH) because the porescan then be fully filled with electrolytic solution at a lower relativepressure of the electrolytic solution, and higher ion conductivity canbe obtained.

[0091] There are no particular restrictions on the method for bondingthe functional groups having an ion exchange function to the organicgroups, but examples include a method in which fuming sulfuric acid,sulfuric anhydride (sulfer trioxide, SO₃), chlorosulfonic acid(chlorosulfuric acid, ClCO₃H), or another such sulfonic oxidant is usedwhen the functional group having an ion exchange function is a sulfonicacid group; a method in which phosphorus oxychloride or another suchphosphorus oxidant is used, or in which chloromethylation is followed byreaction of triethyl phosphite and then hydrolysis, when the functionalgroup having an ion exchange function is a phosphoric acid group; and amethod in which a group whose side-chain group or terminal group is amethyl group is introduced as an organic group, and this methyl group isoxidized, when the functional group having an ion exchange function is acarboxylic acid group.

[0092] There are no particular restrictions on the form of the solidelectrolyte of the present invention obtained as above, but this solidelectrolyte can be made into a thin film and used favorably as a solidelectrolyte membrane. A solid electrolyte thin film can be favorablyobtained in the process of manufacturing the organic/inorganic compositematerial, for example, by coating a glass substrate or the like with asol solution containing a porous material precursor, drying and thenbaking the coating to remove the surfactant and obtain anorganic/inorganic composite in the form of a thin film, and then bondinga functional group having an ion exchange function to the organic groupby one of the methods given above.

[0093] When the solid electrolyte of the present invention is in theform of particles, the solid electrolyte particles of the presentinvention can also be used as a compound electrolyte dispersed inanother electrolyte (hereinafter referred to as a “second electrolyte”).Examples of this second electrolyte include perfluorosulfonic acid,perfluorophosfonic acid, polystyrenesulfonic acid,polyvinylbenzylphosphonic acid, polytrifluorostyrenesulfonic acid,^([3]) and other such homopolymers and copolymers; graft polymersobtained by graft-polymerizing styrene or trifluorostyrene to afluororesin such as ethylene-tetrafluoroethylene copolymers,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, polyvinylidene fluoride, ahexafluoropropylene-vinylidenefluoride copolymer, or an ethylene-chlorotrifluoroethylene copolymer,and performing sulfonation or methylphosphonation; and polysulfonesulfonic acid membranes, polyether ether ketone sulfonic acid membranes,polyparaphenylene derivative sulfonic acid membranes, and other suchhydrocarbon-based polymers. This second electrolyte can be synthesizedby a known method, or a commercially available product may be used. Forinstance, in the case of perfluorosulfonic acid, tetrafluoroethylene anda perfluoroalkylsulfonic acid vinyl ether may be copolymerized underspecific conditions, or a commercially available product such asAciplex, Nafion, or Flemion may be used.

[0094] There are no particular restrictions on the amount in which thesolid electrolyte particles of the present invention are contained inthe composite electrolyte of the present invention, as long as theexcellent ion conductivity thereof is not compromised, but a preferableamount is 40 to 80 wt % of the total amount of the compositeelectrolyte. If the solid electrolyte of the present invention iscontained in the composite electrolyte in an amount below the aboverange, a sufficiently high ion conductivity will tend not to be obtainedif the relative pressure of the electrolytic solution in the atmosphereis less than 1.0, but if the above range is exceeded, the material willtend to be difficult to mold into a film and film strength will tend tobe lower.

[0095] The solid electrolyte and composite electrolyte of the presentinvention having the above structure provide sufficiently high ionconductivity at a lower temperature than with a conventional solidelectrolyte such as stabilized zirconia, even if the relative pressureof the water vapor in the atmosphere is less than 1.0, and can be usedfavorably in applications such as the electrolytic membranes solidpolymer-type fuel cells or oxide-type fuel cells, the electrolyte usedin completely solid cells, sensors, and so forth. There are noparticular restrictions on the electrolytic solution used in the presentinvention, but specific examples include water, alcohol, pyridine, andimidazole, of which the use of water is preferred. Nor are there are anyparticular restrictions on the usage conditions for the solidelectrolyte and composite electrolyte of the present invention, but itis preferable for the relative pressure of the electrolytic solution tobe from 0 to 70%, and for the usage temperature to be between 0 and 100°C.

EXAMPLES

[0096] The present invention will now be described in more specificterms through on the basis of examples and comparative examples, but thepresent invention is not limited in any way by the following examples.

Example 1 Production of Porous Particles

[0097] 16.665 g (47.88 mmol) of octadecyltrimethylammonium chloride(C₁₈H₃₇N(CH₃)₃Cl, hereinafter referred to as “C₁₈TMA”), 500 g of ionexchange water, and 40 g of a 6N NaOH aqueous solution (NaOH content:200 mmol) were put into a 1000 mL pear-shaped flask and stirred at 50 to60° C., which gave a transparent solution. This solution was cooled toroom temperature, after which it was briskly stirred while 20 g (49.67mmol) of 1,4-bis(triethoxysilyl)benzene (hereinafter referred to as“BTEB”) was added, then an ultrasonic treatment was performed for 20minutes while the flask was shaken by hand, which dispersed the BTEB inthe solution. The reaction mixture thus obtained was allowed to standfor 20 hours at 95 to 98° C., which produced a white precipitate. Thereaction mixture was filtered without being cooled, and the precipitatewas recovered, which yielded 8.22 g of porous particle precursorcontaining a surfactant.

[0098] Next, 1 g of the porous particle precursor obtained above wasdispersed in 250 mL of a hydrochloric acid-ethanol mixed solutioncontaining 36 wt % hydrochloric acid, and then stirred for 8 hours at70° C. The precipitate was then recovered by filtration, washed with 250mL of anhydrous ethanol, dried with forced air, and then vacuum dried atroom temperature and 10⁻² to 10⁻³ Torr, in that order, which gave thetargeted porous particles.

[0099] The porous particles thus obtained were measured by ¹³C-NMR and²⁹Si-NMR, subjected to X-ray analysis, measured for nitrogen adsorptionisotherm, and observed under an electron microscope. FIGS. 1 and 2 showthe X-ray diffraction patterns obtained, FIG. 3 shows the nitrogenadsorption isotherm, and FIG. 4 shows the pore diameter distributioncurve. The X-ray diffraction pattern in FIG. 2 is a detail view of theregion where the 2θ of the X-ray pattern shown in FIG. 1 is 1 to 10.

[0100] The above measurement results confirmed that the obtained porousparticles had two-dimensionally hexagonal meso pores, that the skeletonthereof was made up of —C₆H₄—Si₂O₃—, and that the surfactant had beencompletely removed. It was also found that the mean pore diameter of theporous particles was 2.8 nm, the BET specific surface area was 850 m²/g,the pore volume was 0.63 cm³, and the thickness of the pore walls was2.5 nm.

[0101] Production of Solid Electrolyte (Sulfonylation)

[0102] 23 g of 50 mass % fuming sulfuric acid was added to 0.5 g of theabove-mentioned porous particles and stirred for 5 hours at 95 to 105°C. The reaction mixture was cooled to room temperature, after whichethanol was added so as to decompose the excess fuming sulfuric acid.The precipitate in the reaction mixture was recovered by filtration andwashed with water, after which ion exchange water was added and thesystem was boiled for 1 hour. The system was then stirred overnight in6N hydrochloric acid, and the precipitate obtained by filtration wasvacuum dried at room temperature and 10⁻² to 10⁻³ Torr to obtain thetargeted solid electrolyte.

[0103] The solid electrolyte thus obtained was subjected to sodiumhydroxide titration by the following procedure. 50 mg of solidelectrolyte was thoroughly vacuum dried at 70° C. and 10⁻² to 10⁻³ Torr,after which it was soaked overnight in a 10 wt % sodium chloride aqueoussolution. Titration was then performed using a 0.05N sodium hydroxideaqueous solution, and the hydrogen ion (H⁺) equivalent was measured. Thetitration curve thus obtained is shown in FIG. 5. In FIG. 5, curve a isthe correlation between the amount of 0.05N sodium hydroxide aqueoussolution added dropwise and the pH, and curve b is the correlationbetween the titer of a 0.05N sodium hydroxide aqueous solution and d(pH)/dV (pH) (the value obtained by differentiating the pH with thetiter V). As shown in FIG. 5, it was confirmed that 5.5×10⁻⁴ eq/ghydrogen ions were present in the obtained solid electrolyte. Theseresults suggest that 14.3% of the phenylene groups of the porousmaterial skeleton (—C₆H₄—Si₂O₃—) were sulfonylated, forming a skeletonexpressed by —O_(1.5)Si—C₆H₃(SO₃H)—SiO_(1.5)—.

[0104] The above-mentioned solid electrolyte was also measured by X-raydiffraction and nitrogen adsorption isotherm methods. The resultingX-ray diffraction patterns are shown in FIGS. 1 and 2, the nitrogenadsorption isotherm is shown in FIG. 3, and the pore diameterdistribution curve is shown in FIG. 4. These results confirmed that themean pore diameter of the solid electrolyte was 2.8 nm, the BET specificsurface area was 760 m²/g, the pore volume was 0.50 cm³, and thethickness of the pore walls was 2.5 nm, and that a uniform meso porousstructure was maintained even after the introduction of sulfonic acidgroups into the pores. In the X-ray diffraction patterns of the porousparticles and the solid electrolyte shown in FIG. 1, three peaks areseen at 2θ=11.6, 23.5, and 35.5, and these results suggest that thebenzene rings that make up the skeleton of the solid electrolyte (orporous particles) are included in the pore walls, and a regularstructure is present within the pore walls.

Example 2

[0105] A solid electrolyte was produced and sodium hydroxide titrationwas performed in the same manner as in Example 1, except that 30 g of 60mass % fuming sulfuric acid was used instead of the 23 g of 50 mass %fuming sulfuric acid used in Example 1, and the reaction was conductedfor 5.5 hours at 75 to 85° C. As a result, it was confirmed that3.2×10⁻⁴ eq/g hydrogen ions were present in the obtained solidelectrolyte. These results suggest that 8.3% of the phenylene groups ofthe porous material skeleton (—C₆H₄—Si₂O₃—) were sulfonylated, forming askeleton expressed by —O_(1.5)Si—C₆H₃(SO₃H)—SiO_(1.5)—.

Example 3

[0106] A solid electrolyte was produced and sodium hydroxide titrationwas performed in the same manner as in Example 1, except that 30 g ofsulfuric anhydride (SO₃) was used instead of the 23 g of 50 mass %fuming sulfuric acid used in Example 1, and the reaction was conductedfor 5.2 hours at 40° C. As a result, it was confirmed that 1.1×10⁻⁴ eq/ghydrogen ions were present in the obtained solid electrolyte. Theseresults suggest that 2.9% of the phenylene groups of the porous materialskeleton (—C₆H₄—Si₂O₃—) were sulfonylated, forming a skeleton expressedby —O_(1.5)Si—C₆H₃(SO₃H) —SiO_(1.5)—.

Example 4

[0107] A solid electrolyte was produced and sodium hydroxide titrationwas performed in the same manner as in Example 1, except that 30 g of amixed solution of 50 mass % sulfuric anhydride (SO₃)/tetrachloroethylene (CHCl₂CHCl₂) was used instead of the 23 g of 50mass % fuming sulfuric acid used in Example 1, and the reaction wasconducted for 5.5 hours at 50 to 60° C. As a result, it was confirmedthat 1.2×10⁻⁴ eq/g hydrogen ions were present in the obtained solidelectrolyte. These results suggest that 3.1% of the phenylene groups ofthe porous material skeleton (—C₆H₄—Si₂O₃—) were sulfonylated, forming askeleton expressed by —O_(1.5)Si—C₆H₃(SO₃H)—SiO_(1.5)—.

Comparative Example 1

[0108] A polymer solid electrolyte membrane (composed ofperfluorosulfonic acid (trade name Nafion 112, made by du Pont) wasevaluated for its water vapor adsorption characteristics (this test willbe discussed below) as Comparative Example 1.

Comparative Example 2

[0109] The porous particles obtained in Example 1, but in which nosulfonic acid groups had been introduced into the pores, were evaluatedfor their water vapor adsorption characteristics (this test will bediscussed below) as Comparative Example 2.

[0110] Water Vapor Adsorption Characteristics Test

[0111] The amount of water vapor adsorption was measured for the solidelectrolytes obtained in Example 1 and Comparative Example 1 and for theporous particles obtained in Comparative Example 2, when water vaporwhose relative pressure was controlled to a specific value was broughtinto contact [with the solid electrolyte or porous particles] untilsaturation was reached at 25° C. The water vapor adsorption isothermobtained by this measurement is shown in FIG. 6.

[0112] As shown in FIG. 6, with the solid electrolyte of Example 1, itwas confirmed that the amount of water vapor adsorption increasedmarkedly when the relative pressure of the water vapor went over 0.45,and that when the relative pressure of the water vapor was 0.6, thewater vapor adsorption was 0.45 g, which corresponds to 90% of the porevolume, and the pores were fully filled with water even when therelative pressure of the water vapor was less than 1.0. With the porousparticles of Comparative Example 2, an increase in the relative pressureof the water vapor was again accompanied by a marked increase in watervapor adsorption, but the reason behind this was that the relativepressure of the water vapor was over 0.6. These results suggest that theintroduction of sulfonic acid groups into the pores has the effect ofenhancing the water vapor adsorption characteristics.

[0113] In contrast, with the solid electrolyte in Comparative Example 1,the adsorption of water vapor was insufficient even when the relativepressure of the water vapor was over 0.9, confirming that adequate ionconductivity cannot be obtained if the relative pressure of the watervapor is less than 100%.

Examples 5 to 8

[0114] Measurement of Conductivity

[0115] The solid electrolyte of Example 1 was mixed in a specificproportion with an ethanol solution (perfluorosulfonic acid content: 5%)of perfluorosulfonic acid (Nafion, made by du Pont) to form a paste.This paste was dried, after which it was molded in a tablet moldingmachine at a pressure of 1100 kg/cm² to obtain pellets with a diameterof 10 mm. The pellets of Examples 5 to 8 were produced here by varyingthe proportional content of the solid electrolyte ([amount of solidelectrolyte added]/[combined amount of solid electrolyte andperfluorosulfonic acid added]) so as to be 0.60, 0.80, 0.89, and 0.91,respectively.

[0116] Each of the pellets thus obtained was sandwiched between twoperfluorosulfonic acid membranes (Nafion 112, made by du Pont), theproduct of which was mounted in a conductivity measurement cell(electrode: platinum black plated disc coated with a perfluorosulfonicacid (Nafion) solution, electrode diameter: 10 mm), and the electrodeswere pressed on at a pressure of 32 kg/cm². This cell was immersed inpure water, and the AC resistance at 1 kHz was measured with an LCRmeter to find the electrical conductivity. The results thus obtained aregiven in Table 1.

Comparative Example 3

[0117] Porous particles were produced in the same manner as in Example1, after which everything was carried out the same as in Example 5,except that the step of producing a solid electrolyte (sulfonylationstep) was not performed, and instead [these porous particles] weredirectly mixed with a persulfonic acid/ethanol solution, which producedpellets containing porous particles in a proportion (calculated byweight) of 0.60, and the conductivity thereof was measured. The resultsthus obtained are given in Table 1. TABLE 1 Solid electrolyteproportional Conductivity content [S/cm] Example 5 0.60 0.021 Example 60.80 0.017 Example 7 0.89 0.017 Example 8 0.91 0.015 Comp. Example 3(0.60)* 0.0014

[0118] As shown in Table 1, sufficiently high conductivity was attainedwith the pellets of Examples 5 to 8, whereas the conductivity wasextremely low with the pellets of Comparative Example 3. These resultssuggest that sulfonic acid groups play an active part in themanifestation of conductivity. Also, since a great deal of cracking andsplitting was observed after measurement in the pellets of Examples 5 to8, it is believed that the conductivity of the solid electrolyteparticles themselves is higher than indicated by the measurement valuesgiven in Table 1.

[0119] Industrial Applicability

[0120] As described above, with the solid electrolyte of the presentinvention, even if the relative pressure of the water vapor in theatmosphere is less than 1.0, it is still possible to achieve asufficiently high ion conductivity at a lower temperature than with aconventional solid electrolyte such as stabilized zirconia.

1. A solid electrolyte, having pores with a mean pore diameter of 1 to30 nm, comprising: an organic/inorganic composite material having: askeleton comprising a metal atom, an oxygen atom bonded to the metalatom, and an organic group having at least one carbon atom bonded to themetal atom or the oxygen atom; and a functional group having an ionexchange function and bonded to the organic group inside the pores. 2.The solid electrolyte according to claim 1, wherein the functional grouphaving an ion exchange function is at least one selected from the groupconsisting of sulfonic acid groups, phosphoric acid groups, andcarboxylic acid groups.
 3. The solid electrolyte according to claim 1 or2, wherein the skeleton is composed of an organic group having at leastone carbon atom, two or more metal atoms bonded to the same or differentcarbon atoms in the organic group, and at least one oxygen atom bondedto the metal atom.
 4. The solid electrolyte according to any of claims 1to 3, wherein the solid electrolyte is in the form of a thin film.
 5. Acomposite electrolyte produced by dispersing the solid electrolyteaccording to any of claims 1 to 3 which is a first electrolyte in theform of particles, in a second electrolyte.